Those sounds prior to the explosion
Close your eyes and just listen to the tape of the explosion, from the screeching sound to the big bang.
What it sounds like exactly is something gets loose near the top of the strongback, drops down inside there - hitting the structure (and possibly Ox tubing) a few times - then lands on the launch pad. Unfortunately for SpaceX, it seems they didn't have many sensors outside of the rocket to find out what it was. But I'd take a good look at the debris down there and search for something that looks like it was damaged cold, before the explosion; that should be their trace.